Submitted to: Physiologia Plantarum
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/1/2003
Publication Date: 2/24/2003
Citation: SALVUCCI, M.E., CRAFTS-BRANDNER, S.J. INHIBITION OF PHOTOSYNTHESIS BY HEAT STRESS: THE ACTIVATION STATE OF RUBISCO AS A LIMITING FACTOR IN PHOTOSYNTHESIS.. PHYSIOLOGIA PLANTARUM. 2003. 179-186. Interpretive Summary: In the process of photosynthesis, plants convert light into chemical energy. The energy produced by photosynthesis is then used to synthesize sugars and other foodstuffs. Heat stress inhibits photosynthesis, reducing the overall yield of the plant. Previous research from this research group identified an enzyme called Rubisco activase as the component of photosynthesis that is most sensitive to inhibition by heat. Rubisco activase is a regulatory enzyme that controls the activity of the major carbon dioxide-fixing enzyme in plants. In this review, we discuss recent data that has elucidated the biochemical basis for inhibition of photosynthesis by moderate heat stress. Specifically, we show how reduced activase activity, coupled with faster deactivation of Rubisco together contribute to an inhibition of photosynthesis under heat stress. This information eventually can be used to make changes that improve the activity and stability of Rubisco activase in order to improve the efficiency of photosynthesis under heat stress.
Technical Abstract: The low affinity of Rubisco for CO2 and its dual nature as an oxygenase limit the possible increase in net photosynthesis with temperature. For cotton, comparisons of measured rates of net photosynthesis with predicted rates indicate that direct inhibition of photosynthesis occurs at temperatures higher than about 30ºC. Inhibition of photosynthesis by moderate heat stress (i.e., 30 to 42ºC) is generally attributed to reduced rates of RuBP regeneration caused by disruption of electron transport activity. However, measurements of chlorophyll fluorescence and metabolite levels at air-levels of CO2 indicate that electron transport activity is not limiting at temperatures that inhibit CO2 fixation. Instead, recent evidence shows that inhibition of net photosynthesis correlates with a decrease in the activation state of Rubisco in both C3 and C4 plants and that this decrease in the amount of active Rubisco can fully account for the temperature response of net photosynthesis. Biochemically, the decrease in Rubisco activation can be attributed to 1) more rapid deactivation of Rubisco caused by a faster rate of dead-end product formation and 2) slower reactivation of Rubisco by activase. The net result is that as temperature increases activase becomes less effective in keeping Rubisco activated. In this opinionated review, we discuss how these processes provide a mechanism for the inhibition of photosynthesis by moderate heat stress.